Autocatalysis Lab

Materials:

4 grams potassium chlorate

12.5 grams sodium sulfite

5 milligrams bromophenol blue indicator

3 M sulfuric acid (add 5 mL of concentrated sulfuric acid to 18 mL of distilled water)

100 mLwater

100 mL graduated cylinder

two beakers to mix

Safety goggles

Procedure:

1. Place 50 mL of water in a beaker.

2. Add 4 grams of potassium chlorate, 12.5 grams of sodium sulfite, and about 5 milligrams of the bromophenol blue indicator. Do not mix these solids. Dissolve them in the water.

3. In a second beaker, add 4 mL of 3 Molar sulfuric acid to 50 mL of water.

4. Slowly, with constant stirring, add the diluted acid from the second beaker to the solution in the first beaker. Stir until everything dissolves. The solution should be blue-violet.

5. Fill a 100 mL graduated cylinder with the solution.

6. Carefully add two droppers full of 3 Molar sulfuric acid solution to the top of the liquid in the cylinder.

7. Soon a yellow color will appear at the top of the solution, and a yellow blue interface will form.

8. Observe for several minutes as the yellow-blue interface moves down the graduated cylinder.

Reactions:

1. This reaction is a redox reaction:

ClO₃^- (aq) + 3 HSO₃^- (aq) ---> Cl^- (aq) + 3SO₄^-2 (aq) + 3H⁺ (aq)

HSO₃^- is responsible for the blue color

SO₄^-2 is responsible for the yellow color.

The addition of the sulfuric acid is considered to catalyze the reaction.

Have students perform the lab in groups and observe the reaction closely.

Questions:

1. Why does the yellow-blue interface move down the cylinder?

2. Define autocatalytic

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